Photodimerization in Crystalline 9-Cyanoanthracene

Abstract

ROBERTSON¹ has pointed out that the type of crystal structure adopted by an aromatic molecule appears to depend on the molecular shape. Thus, those in the form of elliptical planes crystallize with an anthracene-type (A) structure on which weak coupling between molecules in the electronically excited state is responsible for a slight shift in the fluorescence spectrum with respect to the molecular spectrum, and the vibrational structure is largely preserved. Disk-shaped molecules such as pyrene and coronene, on the other hand, exhibit strong coupling with a close-packed type B structure in which pairs of molecules present as a structural ‘unit’ or as an imperfection are responsible for the emission of the characteristic excimer band, red-shifted by some 6,000 cm⁻¹ from the molecular spectrum and devoid of vibrational structure².